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<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./sequences.html" class="navigation-link">Sequences</a> / <a href="./intrinsic-metafunctions.html" class="navigation-link">Intrinsic Metafunctions</a> / <a href="./value-type.html" class="navigation-link">value_type</a></td>
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<div class="section" id="value-type">
<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1447">value_type</a></h1>
<div class="section" id="id371">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename Sequence
    , typename X
    &gt;
struct <a href="./value-type.html" class="identifier">value_type</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
</div>
<div class="section" id="id372">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>Returns the <a class="reference internal" href="./associative-sequence.html#value-part">value</a> that would be used for element <tt class="literal"><span class="pre">X</span></tt> in <tt class="literal"><span class="pre">Sequence</span></tt>.</p>
</div>
<div class="section" id="id373">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/value_type.hpp" class="header">boost/mpl/value_type.hpp</a>&gt;
</pre>
</div>
<div class="section" id="id374">
<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3>
<p><a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a></p>
</div>
<div class="section" id="id375">
<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
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<tr><th class="head">Parameter</th>
<th class="head">Requirement</th>
<th class="head">Description</th>
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<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">Sequence</span></tt></td>
<td><a class="reference internal" href="./associative-sequence.html">Associative Sequence</a></td>
<td>A sequence to query.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">X</span></tt></td>
<td>Any type</td>
<td>The type to get the <a class="reference internal" href="./associative-sequence.html#value-part">value</a> for.</td>
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</tbody>
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</div>
<div class="section" id="id376">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<p>For any <a class="reference internal" href="./associative-sequence.html">Associative Sequence</a> <tt class="literal"><span class="pre">s</span></tt>, and an artibrary type <tt class="literal"><span class="pre">x</span></tt>:</p>
<pre class="literal-block">
typedef <a href="./value-type.html" class="identifier">value_type</a>&lt;s,x&gt;::type v;
</pre>
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<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><p class="first"><tt class="literal"><span class="pre">x</span></tt> can be put in <tt class="literal"><span class="pre">s</span></tt>.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first"><tt class="literal"><span class="pre">v</span></tt> is the <a class="reference internal" href="./associative-sequence.html#value-part">value</a> that would be used for <tt class="literal"><span class="pre">x</span></tt> in <tt class="literal"><span class="pre">s</span></tt>.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Postcondition:</th><td class="field-body"><p class="first">If
.. parsed-literal:</p>
<pre class="literal-block">
<a href="./has-key.html" class="identifier">has_key</a>&lt; s,<a href="./key-type.html" class="identifier">key_type</a>&lt;s,x&gt;::type &gt;::type
</pre>
<p>then
.. parsed-literal:</p>
<pre class="literal-block">
<a href="./at.html" class="identifier">at</a>&lt; s,<a href="./key-type.html" class="identifier">key_type</a>&lt;s,x&gt;::type &gt;::type
</pre>
<p class="last">is identical to <tt class="literal"><span class="pre"><a href="./value-type.html" class="identifier">value_type</a>&lt;s,x&gt;::type</span></tt>.</p>
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</div>
<div class="section" id="id377">
<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
<p>Amortized constant time.</p>
</div>
<div class="section" id="id378">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
typedef <a href="./value-type.html" class="identifier">value_type</a>&lt; <a href="./map.html" class="identifier">map</a>&lt;&gt;,<a href="./pair.html" class="identifier">pair</a>&lt;int,unsigned&gt; &gt;::type v1;
typedef <a href="./value-type.html" class="identifier">value_type</a>&lt; <a href="./set.html" class="identifier">set</a>&lt;&gt;,<a href="./pair.html" class="identifier">pair</a>&lt;int,unsigned&gt; &gt;::type v2;

<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; v1,unsigned &gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; v2,<a href="./pair.html" class="identifier">pair</a>&lt;int,unsigned&gt; > ));
</pre>
</div>
<div class="section" id="id379">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./associative-sequence.html">Associative Sequence</a>, <a class="reference internal" href="./key-type.html">key_type</a>, <a class="reference internal" href="./at.html">at</a>, <a class="reference internal" href="./set.html">set</a>, <a class="reference internal" href="./map.html">map</a></p>
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